Stress corrosion cracking (SCC) has led to the critical need for repair or replacement of many components and piping in boiling water reactors throughout the world. Welded joints have historically been the areas most likely to fail due to SCC because of their typically high values of tensile residual stress and their high degree of thermal sensitization in the HAZ. One solution to this problem is to replace components with new material having improvements in chemical composition. Due to the exceedingly high cost of replacing some components, the replacement must be durable. Replacements are generally an installation of newer SCC-resistant material joined to older, SCC-susceptible material, so it is highly desirable even for these cases that the joining process improve the residual stress and microstructural conditions in the older material, since the relatively low thermal efficiency, and the resultant effect of overheating, of conventional joining practices has often been one of the direct causes of the old component's failure.
Thus, there is a need for a mechanized welding process which will produce weld joints having very significantly improved SCC resistance. This can be accomplished using joint designs with deep but very narrow groove widths to minimize the amount of heat put into the weld material, thereby reducing the tensile residual stresses in the vicinity of the weld joint. Another benefit is an improvement in the SCC resistance of the microstructure of the heat affected zones (HAZ) adjacent to the weld.
In addition, there is a need for a welding method which decreases the welding time, and the corresponding man-rem personnel radiation exposure and production costs, associated with work on a "critical path" of an operating nuclear power plant. Conventional welding practices, including those used for field work, have relatively low overall thermal efficiency since a large portion of the heat goes into melting the required large volume of filler wire, rather than into fusing the walls of the joint together. This condition is a direct result of the unnecessarily wide joints used. In contrast, the use of very narrow weld grooves improves productivity due to the higher thermal and volumetric efficiencies of this new method, resulting primarily from the reduced heat input parameters and the reduced-width joint design, respectively.
An approach used in the welding industry to complete narrow groove joints on thicker material when the electrode and/or filler wire stickout beyond its means of support becomes excessive is to make the weld torch assembly as thin as practical, to fit within the joint and to be able to reach near or to the bottom, and then to make the joint width as narrow as possible consistent with this reduced internal torch width. Use of this style of internal torch still results in such a wide joint that other techniques must be resorted to in order to make the filler metal pool wet both sidewalls alternately, such as electrode tip lateral oscillation or magnetic arc lateral oscillation, or use of two or more passes per layer.
The approach of thinning the torch to fit within the joint (and in some cases thinning the viewing device as well) has the severe disadvantage of being limited in the amount of joint width reduction possible according to the reduced size of the torch, which typically includes provisions for an electrode holder, a weld gas cup/nozzle, a wire feed guide nozzle (for non-consumable electrode processes), water cooling flow circuits as required, and sometimes viewing camera optical components as well when used with remotely applied processes. The net result is a joint width which is considerably greater than desired to obtain a minimum weld width and therefore minimum weld volume which can be soundly completed with a minimum of heat input. Achieving these minimum values provides the correspondingly lower tensile residual stresses, reduced heat affected zone size and severity, and shorter filler material deposition time.
An improvement over the foregoing apparatus is to make a flat electrode blade which extends vertically downward into the weld groove from a torch block which remains outside the groove. An electrode blade of reduced thickness facilitates arc welding in a groove of reduced width. In the case of a narrow groove which is shallow, a gas cup may be arranged outside the groove to direct an inert gas or a mixture of inert gases into the groove. The gas cup has a gas lens incorporated therein for ensuring that the gas flow out of the gas cup and into the groove is substantially laminar. However, as the depth of the narrow groove is increased, it becomes increasing difficult to provide adequate gas coverage at the bottom of the groove, overlying the weld puddle. In addition, it is conventional practice to provide a gas cup having a width greater than the width at the opening of the weld groove. The result is that some of the shielding gas from the gas cup does not enter the groove, leading to shielding gas wastage and increased welding costs.